1. Field of the Invention
The present invention generally relates to variable delivery pumps and more specifically pertains to pump output control systems that provide for an infinitely variable volumetric output while minimizing the pump's power consumption.
2. Brief Description of the Prior Art
It is often desirable or even necessary to be able to reduce the volumetric output of a pump and while simply slowing down the pumping speed may be a viable alternative in some applications, such approach is not always an option. For example, the pump's speed may be dictated by an efficiency peak specific to the particular pump design or specific to the particular power system employed to drive the pump. Moreover, the pump is often driven as a peripheral function of a power system and the pump's volumetric output requirements may be completely independent of its driven speed. Consequently, it is most desirable to have the ability to vary a pump's volumetric output independent of its driven speed.
While simply bleeding off pressurized output for return to the low pressure side of a pump serves to reduce the net output of the pumping system, such approach results in gross inefficiencies as the pump's power consumption would remain substantially constant despite reductions in volumetric output. Continually returning pressurized fluid to pump input may additionally result in heating of the fluid being pumped which may in and of itself comprise an undesirable or unacceptable effect.
Gear pumps are often employed in various applications due to their relative affordability and reliability, however attempts to develop a variable delivery gear pump mechanism have met with limited success. In a typical approach, the length of engagement of the gears is varied by axial displacement of one gear so as to offset its position with respect to the other gear. Poor efficiency due to high internal leakage, limitations imposed by gear tooth strength problems at high offset/low partial displacements, and prohibitive manufacturing costs have prevented such designs from gaining success.
In the alternative, control systems have been developed that operate on the discharge fluid stream of pumps in an effort to provide for an adjustable volumetric output in a power efficient manner. Lipinski, U.S. Pat. No. 2,771,844 provides such a system in combination with a gear pump mechanism but the design nonetheless suffers from a number of inherent disadvantages. The Lipinski system relies on a rotating spool valve to alternately divert the pump's output between a discharge line and a return line in a cyclical fashion. The axial position of the spool valve determines the relative dwell times at either port and its axial position is infinitely adjustable. Since the pump encounters no significant resistance while its output is being returned to its low pressure side, power consumption is substantially a function of the volume actually delivered under pressure.
The spool valve configuration employed by Lipinski comprises an axially slidable cylinder rotationally driven by the pump's idler gear. An obliquely oriented groove formed on the cylinder's surface serves to alternately engage a discharge port and a return passage port that are disposed in the bore in which the cylinder rotates while the output side of the gear pump remains in constant communication with the groove. In order to prevent inefficiencies and other undesirable side effects associated with a momentary back flow, the width of the groove and the positions of the two ports are selected such that the discharge line is at no time set into communication with the return line. As a result, the output of the pump is necessarily momentarily completely blocked or trapped twice with every revolution of the spool valve just after the groove moves away from one port and just before it engages the other port. This causes a momentary, extreme build-up in pressure which results in destructive loads, noise, and some loss in efficiency. The fact that only a single pulse of output is delivered with every revolution of the spool valve results in further roughness, noise and vibration. Additional disadvantages associated with the Lipinski design are inherent in the fact that the spool valve simultaneously serves as the idler gear shaft. This subjects the spool valve to substantial side loads, friction and consequently wear which eventually results in an increasing amount of valve leakage. Further, if substantial pressures are involved, the side loads exerted on the valve can render its axial adjustment exceedingly difficult. While Lipinski does provide a variable delivery pumping system, its relatively slow cycling rate, the high internal loads and inefficiencies due to intermittent flow blockage, the resulting undesirable noise and vibration, and the side loads placed on the spool valve comprise substantial disadvantages.